LITHOLOGY. 181 
of it has been broken away. It is, however, as fine a specimen as could 
be desired, to show a combination of two systerns of twinning. When 
revolved between crossed Nicol prisms, this section reaches its maximum 
of darkness when a side is parallel to the plane of vibration of the light. 
The ortho-diagonal therefore lies in the section; and if the section is 
prismatic, then one composition plane is parallel to the clino-diagonal, 
and the other composition plane is the clino-dome, and we have a double 
twinning, which is not uncommonly found in crystals which have devel- 
oped in cavities, but which is very rare in ingrown crystals. The con- 
trasts of color in the section are not very great, for, in a prismatic section 
cut approximately parallel to the ortho-diagonal, neither kind of revolu- 
tion throws the axes of elasticity far away from one another. Baveno 
twins have been found—first, by E. Weiss—in the sanidin of trachytes 
and modern eruptive rocks, but I am not aware of their having been ob- 
served in quartz porphyries, though they have been frequently observed, 
first by Rose, in granite.* 
This porphyry contains odd little concretionary masses of hornblende 
and chlorite, which fill spaces that were perhaps originally empty. The 
mineral grains on the outside are stained yellow by decomposition, and 
thus the bright green minerals within are surrounded by a yellow wreath. 
Crystals of orthoclase, which are pure within and impure without, are 
also found here. The porphyries of this place are stated by Prof. Hitch- 
cock to be plainly eruptive. 
A gray quartz porphyry from New Zealand contains so many large 
crystals of quartz and feldspar, that the porphyritic character is almost 
obliterated; and from this rock to granite is not a wide step. 
A gray quartz porphyry from Waterville possesses a very fine felsitic 
base; and the sections of many of the quartz grains are polygons, which 
indicate the presence of dehexahedrons of quartz that can often be ma- 
croscopically seen. As is usual in such porphyries, these crystals have 
rounded edges. 
This porphyry is a favorable one in which to observe the innumerable 
little cavities which are characteristically present in the quartz of all 
porphyries. When highly magnified, in these cavities a fluid and a bubble 
are almost always seen, and, moreover, a little colorless cube is often 
| & Rosenbusch Physiographie der massigin Gesteene, p. 12. 
